The diaphragm plays a significant role in ventilation, and its dysfunction can result in difficulty weaning from MV [4]. The diaphragm weakness developed rapidly in the first few days of MV [12,13,14]. Growing evidence showed that diaphragm dysfunction contributed to weaning failure and prolonged ventilation [11, 13, 15, 16]. Diaphragmatic function assessment is vital in critically ill patients and this part of the population with invasive MV. It is also important to consider the factors that affect and predict the success of ventilator weaning.
Diaphragm ultrasound can currently be performed at the bedside to monitor the diaphragm movement, the diaphragm thickness, and the thickening rate [8, 17, 18]. However, conventional methods, such as thickness fraction, or caudal displacement assessed by M-mode have limitations, such as angle dependence and translational error. At present, there is no standard quantification for measuring diaphragmatic movement, because the moderate consistency of two-dimensional ultrasound was a bottleneck problem. Our study aimed to provide a novel quantifiable method of diaphragm function analysis.
In this study, the speckle tracking technology was used to measure the diaphragmatic excursion and its velocity. Compared to conventional ultrasound, it automatically chose three to six ROI on the diaphragm when measuring, calculated its speed or displacement in different parts of the diaphragm, and then form a general parameter, which was accurate and comprehensive to evaluate the excursion speed of the entire diaphragm. In addition, the original intention of developing the software was to standardize the measurement of the diaphragm ultrasound, including excursion and velocity.
Pesero et al. discovered the anatomical M-mode which allowed free placement of the cursor to measure diaphragmatic excursion and helped recognize diaphragmatic dysfunction since the conventional analysis line overestimated excursion in cardiac surgical patients[19]. Orde et al. proposed the use of angle-independent M-mode sonography for the assessment of diaphragm displacement, demonstrating that the cursor might not be orientated to the true direction of the diaphragm movement, leading to orientation and translation error[20]. Inspired by the previous studies, we calculated a calibration line during the automatic measurement (Additional file 8 videos 3 and Additional file 9: video 4). Our results suggested that the automatic measurement of diaphragmatic excursion velocity was lower than that obtained by manual measurement, which might be due to the use of the anatomic M-line adjusted algorithm. The abovementioned study suggested that the diaphragmatic excursion measured by conventional M-mode was overestimated [19]. It might partially explain the result of the automatic measurement of velocity. However, it should be emphasized that this was a newly developed software, which still needed to be trained with large sample data to achieve continuous improvement. Overall, the present study exhibited a scenario where diaphragmatic kinetics assessment could be performed via automatic measurement.
In addition, the low excursion and velocity might be contributed to the timing of the ultrasound. Ultrasound was performed before SBT in the present study while the previous literature reported ultrasound data collected during the first 30 min of SBT [11]. Cammarota et al. investigated the diaphragmatic excursion velocity measured with tissue Doppler imaging at the end of the SBT [21]. The result suggested that subjects who developed both extubation failure and success experienced a greater diaphragmatic activation, compared with the result in the present study. Upon MV assistance, diaphragmatic movement and contraction might not require too much effort due to the positive pressure support. Another study indicated that the mode of ventilation affected the preservation of diaphragmatic contraction, as MV support, could partially reverse the muscle atrophy process [22]. It might be mutually verified that the diaphragmatic excursion and velocity were affected by MV. For acutely hospitalized patients ventilated more than 24 h, guideline suggested that the initial SBT be conducted with inspiratory pressure augmentation (5–8 cm H2O) rather than without [23]. Using low-level pressure support or continuous positive airway pressure counteracted the resistance of the breathing circuit. The initial purpose of the diaphragmatic assessment was to predict the extubation success so that to avoid the potentially hazardous effects, such as respiratory muscle fatigue or dyspnea, caused by SBT. Therefore, we chose to perform the diaphragmatic assessment before SBT. Moreover, all of the patients had spontaneous breathing with the pressure support of 10–12 cm H2O. We believed that it simulated SBT with inspiratory pressure augmentation, to a certain extent.
Expert consensus recommended diaphragmatic movement ultrasound measurement and emphasized the importance of context-specific or outcome-related cut-off values[1]. The results of the present study showed that the correlation between manual measurement and automatic speckle tracking measurement was high. Follow-up data on clinical adverse outcomes were collected to validate the prognostic value. In the present study, a 43.18% (38/88 patients) incidence of weaning failure was observed. ROC curve analysis showed that a mean excursion ≤ 1.3 cm, and a mean velocity ≤ 0.3 cm/s represent possible predictors for prolonged weaning. The AUROC curves for these variables were 0.782 and 0.679, respectively, our results also suggested that a mean excursion ≤ 1.0 cm was predictive of weaning failure (AUROC = 0.659), while a mean excursion ≤ 1.0 cm prognosticated in-hospital death/withdrawal of treatment (AUROC = 0.614). The cut-off value was consistent with the diagnostic criteria of diaphragmatic dysfunction[24].
The software calculations are based on an algorithm patented, which is not open to the public. The present study provided a pilot vision toward a novel measurement for diaphragm ultrasonography in research and daily practice, compared with the currently used techniques.
There are limitations to the present study. Primarily, given the small sample size in our pilot study, a larger, multicentered study could be useful to validate the role of the current software module. Second, speckle tracking could be applied to automatically measure the thickness and changing rate of the diaphragm. The rate of change in the thickness of the diaphragm, plus the excursion and velocity data that has been achieved so far, may be valuable in the evaluation of the diaphragm function. A combination of several parameters might provide multiple dimensions and enhance predictive power.